Photographic silver halide emulsions are generally prepared by precipitation of the silver halide in the presence of a carrier or binder, generally gelatin, wherein the silver halide grains are formed by the interaction of a water-soluble silver salt, such as silver nitrate, and a water-soluble halide, such as potassium bromide. The formation of the silver halide grain is usually accompanied by the liberation of counterions which must be removed in order to render the surface of the silver halide grain available for efficient chemical sensitization and to coat the grains without the risk of formation of crystals of the counterions which would deform the emulsion layer and render it unsuitable for photographic use.
In order to avoid the deleterious effects of the counterions, extensive removal techniques well known to the art are employed. These washing operations are varied and extensive and occupy a large proportion of the time and equipment employed in emulsion manufacture. The removal of the counterions by a washing procedure is one of the most critical phases of emulsion manufacture since the quality or even the usefulness of the emulsion depends upon this procedure. The term "grain" as used herein refers to a crystalline particle of silver halide and should be understood to include particles of any composition of silver halide with any mixture of crystal habits.
From the foregoing, it will be noted that the formation of the silver halide grains and the sensitization takes place in the presence of a binder material. Gelatin is the most commonly used binder material for silver halide, but other materials such as synthetic polymers are also employed. It is a requirement of the binder material that it permit the growth of silver halide grains at a controllable rate. The binder material must also be capable of being noodled or flocculated to permit washing of the emulsions to remove unwanted counterions and excess salts. It is also a requirement that the binder material allow the various sensitization processes to take place. A further requirement is that the binder prevent agglomeration of the silver halide grains and not be salted out by the counter ions present. These requirements disqualify a large number of synthetic polymeric materials from being employed in silver halide emulsions when otherwise they may possess some properties desired in such employment. For example, some polymers are good for grain growing but not for the washing step, and vice versa. In addition, because the reaction forming the silver halide grains takes place in the presence of the binder, starting materials and reaction products other than silver halide are entrapped therein, which contribute to the necessity of the above-mentioned extensive washing procedures.
A novel method for forming emulsions has now been found which is not susceptible to the deficiencies of the prior art and which circumvents the above limitations by forming silver halide grains without substantially increasing the concentration of counter-ions and therefore eliminates wash steps and removes many restrictions on polymers.